Synthetic resin containers of the shapes of bottles and cups formed by using synthetic resins such as polyethylene terephthalate and polyolefin, have now been widely put to practical use as containers for beverages and foods.
Such synthetic resin containers are obtained by injection-forming a synthetic resin such as polyethylene terephthalate or the like, or by forming a preformed article (usually called preform) by compression-forming a synthetic resin material in a molten state and, thereafter, biaxially stretch-blow-forming the preform. The containers can be further formed by blow-forming a hollow parison of a polyolefin such as polypropylene or polyethylene, or of a synthetic resin using, as an intermediate layer, a gas-barrier resin such as ethylene-vinyl alcohol, or by compression-forming a synthetic resin material in a molten state.
Here, in conducting the compression forming, the synthetic resin of a molten state (hereinafter called molten resin) is extruded from an extrusion opening formed in an extrusion die head mounted on an extrusion means, the extruded molten resin is cut from the extrusion opening and is transferred to a required position by a cutting mechanism and a holding mechanism that move across the extrusion opening, and is fed to a required portion in a compression metal mold. Here, to form a desired article, it is important that the molten resin cut by the cutting mechanism is correctly fed to the desired portion in the compression metal mold via the holding mechanism and, therefore, the molten resin must be extruded from the extrusion opening in a state of hanging down substantially vertically.
In extruding the molten resin, however, the molten resin extruded from the die head is often bent due to pressure and rate of extrusion, kind of the resin, structure of the extrusion passage in the die head and unevenness in the temperature of the molten resin. The molten resin that is cut and transferred in a bent manner cannot be correctly fed into the compression metal mold and makes it difficult to obtain the preform or the container as desired.
As a technology for correcting the bending of the molten resin, a patent document 1 discloses a correction method of correcting the bending of a resin in a molten state extruded from an extrusion opening formed in an end of an extrusion die head by inserting a resin flow control pin in a resin passage in the extrusion die head of an extruder, and by controlling the flow of the resin by adjusting the position of the resin flow control pin. At the time of adjustment according to the above correction method, the position of the molten resin flow control pin must be adjusted by hand while operating the extruder in a state where the molten resin mass no longer cut and is no longer transferred to the compression metal mold.
Further, a patent document 2 discloses a molten resin feeding apparatus equipped with a monitoring means capable of monitoring, from the lower side in the vertical direction, the extruded state of the molten resin extruded from the extrusion opening to fully accurately analyze the extruded state during the period of until the state of extruding the molten resin is stabilized without hindering the fall of the molten resin from the extrusion opening onto the holding means. The monitoring means includes an imaging means, and is moved away from the lower side of the opening in case of abnormal operation.
The above two documents, further, disclose the constitutions for cutting the molten resin extruded from the extrusion opening by a cutting mechanism to form a drop (material for compression forming) of a predetermined amount and for feeding the molten resin by inserting it in a dent of a female forming mold.
Further, a patent document 3 discloses an extrusion-forming method for controlling the thickness profile of the formed article to be constant by detecting the thickness profile of the formed article and by controlling the temperature distribution of an extrusion metal mold based on the detected results, wherein the extrusion metal mold constituted by a cylindrical outer die and an inner die is provided with a plurality of heating means and cooling means, and the individual heating means or the cooling means are separately controlled.
The method of the patent document 3 is to prevent the deviation in the thickness of the formed article at the time of extruding the cylindrical formed article or is to control the thickness of the formed article, and cannot be applied to the solid and molten resin to which the present invention is concerned.